Did you use an OEM drivebelt? I had mine replaced at 9 years old when the voltage regulator in my alternator went bad and the mechanic replaced it with a Dayco belt which squealed like crazy for no reason a little over a year later.

Over the 4-5 years I've owned my current yaris, I have replaced the drive belt 4 times (the 4th one last weekend). Each time because it starts squealing at startup, even after re-tightening. Each time with a different brand from the local shops. Cheap or expensive, it makes no difference. This last time I just took it to the shop and had them do it. At least now if it starts doing it again I can just take it back to them and say "fix your crap". Lets hope 4th time is the charm.